The present invention generally relates to a signal transmission method. More particularly, the present invention is concerned with a method suited profitably for monitoring the state of a power distribution system in which a master station and a plurality of slave stations are interconnected by way of a power distribution line and in which transfers of information are performed sequentially between the master station and a plurality of the slave stations for the purpose of monitoring the state of the distribution system.
As an information transmission system for detecting and grasping the states of pole-mounted switches being distributed in a power distribution system, there can be mentioned a power line signal transmission system in which the power distribution system itself is used as a signal transmission line and a wired transmission system in which a dedicated transmission line is employed, as is described, for example, in "HITACHI HYORON", Vol. 65, No. 6 (1983-6) and others.
In a method of collecting the information concerning the state of the power distribution system by resorting to the use of the transmission system mentioned above, there is generally adopted a polling technique according to which a master station calls sequentially a plurality of slave stations placed under the control of the master station, wherein the state of the whole distribution system is grasped on the basis of the information sent sequentially from the slave stations.
The polling method requires an amount of transmission time which is in proportion to the number of the slave stations for obtaining the information from all the slave stations because the master station has to interrogate sequentially the slave stations one by one. By way of examples, when communication between the master station and one slave station requires a time of t seconds, then the time taken for communication between the master station and n slaves stations amounts to t.times.n seconds.
In general, a change of the state of the switches installed in the power distribution system (which is brought about by system failure, accidents and for other reasons) does not take place frequently. It is however a matter of great importance to detect as early as possible the change in the state of the switches in order to avoid inconvenience of the consumers brought about by the service interruption by taking the measures for the restoration speedily. As an approach for solving this problem, there may be mentioned, for example, an emergency interrupt transmission system disclosed in JP-A-61-61540.
In the emergency interrupt transmission system, a signal or information sent out by a given station is transmitted simultaneously to all the stations installed in the transmission system of concern, wherein all the stations are implemented on the basis of the same transmission principle, as with the case of the power line transmission system. The emergency interrupt transmission system is characterized in that a slave station which detects a change of state of the associated switch and hence must inform the situation immediately to the master station is allowed to issue an interrupt request in synchronism with the calling or polling of the master station for the slave stations. Consequently, the signal sent out from the master station may conflict with the interrupt signal issued by the slave station which detected the change of the state, whereby the signal outputted by the master station may undesirably be prevented from being correctly transmitted as intended by the master station. In such case, the slave station as called may decide that the incoming signal from the master station is invalid and issue no reply. On the other hand, the master station may decide that the interrupt signal overlaps the signal sent out therefrom and assume the send-by state waiting for the information from the slave station which issued the interrupt signal. Subsequently, the slave station which detected the change of the state can transmit the relevant information toward the master station. The prior art system suffers a drawback in that because the master station may erroneously take the error in the signal sent therefrom for the occurrence of emergency information transmission from an unspecified slave station, there may arise undesirably an unnecessary stand-by state upon occurrence of transmission failure (such as caused by a lighting stroke to the distribution system), while the error due to the failure may erroneously be recognized as emergency information.